In a discharged lead-acid cell, the active material of both plates is lead sulfate, and the electrolyte is mostly water, a very weak sulfuric acid solution.
When being charged, the cell is connected to a DC electrical source with electrical pressure higher than that of the cell, since it must act like an electron pump forcing electrons from the positive plates to the negative plates.
At the negative plates, sulfate is discharged. More sulfuric acid forms, and the plate changes into sponge lead. At the same time, lead peroxide is formed at the positive plates, which restores the cell's electrical potential.
The charging process increases the amount of acid in the electrolyte, making the electrolyte stronger. When further charging no longer makes the electrolyte stronger, charging is complete.
Connecting a lead acid battery to a load causes chemical changes as the battery discharges.
At the positive plate, sulfate from the electrolyte joins with lead to form lead sulfate, and oxygen from the plate joins the hydrogen from the electrolyte to form water. Lead sulfate also forms at the negative plate, as sponge lead forms with sulfate from the electrolyte.
Overall, the percentage of acid in the electrolyte falls, and the percentage of water rises, which reduces the strength of the electrolyte. As the cell discharges, the plates develop the same composition, which reduces the potential of the cell.
Recharging the battery again restores the difference between its sets of plates.
